1. Field of the Invention
The present invention relates generally to the lamination of safety glass products, usually comprising one or more layers of interlayer material that adhere two or more glass sheets together. The term "glass sheets" includes silica glass compositions and rigid plastic sheets composed of well known substitutes for glass such as polycarbonates, acrylics, polyurethanes, polyesters and the like. The glass sheets laminated together may comprise one or more sheets of heat strengthened glass or at least one of the glass sheets may contain a coating for decorative or other functional purpose, such as serving as a heat screen.
Safety glass is conventionally made by sandwiching a layer of plastic material between two sheets of glass. The laminate usually includes an interlayer of plasticized polyvinyl butyral which bonds the glass sheets together. To obtain the intimate contact at the interfacial surfaces, which is necessary to obtain a suitable lamination, air is first excluded from the interfacial surfaces between the sheets forming the assembly to be laminated by pre-pressing, followed by final lamination.
In one pre-pressing technique, the assembly is passed while heated between a pair of rubber rolls. Another technique for pre-pressing is to apply a flexible open wall ring about the periphery of the assembly and evacuate the interfacial surfaces through the peripheral chamber formed by enclosing the periphery of the assembly within the lips of the open wall ring and heating the assembly to bond the sheets together sufficiently to prepare the assembly for final lamination in an autoclave. The latter step is performed by exposing the assembly to either air or oil at an elevated temperature and pressure in a large and expensive pressurized autoclave. In the autoclave, the assembly of two relatively rigid plies of glass separated by a relatively soft interlayer is subjected to heat and pressure for a suitable length of time. As a result of the exposure in the autoclave, an irreversible deformation of the interlayer takes place. This causes the interfacial surfaces of the interlayer to conform and to develop a strong adhesive bond to the interfacial surfaces of the glass sheets. However, the autoclave process is expensive and wasteful of energy. It would be preferable to develop a less expensive process to fabricate laminated glass products, particularly a technique that avoids the necessity for a prepressing step followed by final lamination under autoclave conditions.
Laminated safety glass units used in overhead glazing and in sloped glazing installations are composed of glass sheets that are heat strengthened, particularly glass sheets whose heat strengthening is accomplished while the glass sheets are conveyed along a roller-type conveyor. Such heat strengthening involves heating the glass sheet to a temperature at which the surfaces soften somewhat followed by a cooling step in which the sheets develop a stress gradient comprising a compression stress at the surface and an interior tension stress. Since the softened surfaces of the glass sheet roll over the roller conveyor during heat strengthening, they develop a wave and, therefore, are not as flat as sheets that require less heat during their processing, such as sheets that are annealed or brought to a lower stress configuration.
When glass sheets are thermally processed while conveyed on a roller conveyor, they tend to develop a warp in which one major surface becomes convexly curved and the other major surface concavely curved, depending upon the temperature cycle to which the opposite major surfaces are exposed during the thermal treatment required for heat strengthening. The warp that results is a function of the magnitude of the stress gradient developed through the thickness of the glass sheets as a result of the heat strengthening operation.
In addition, a rectangular glass sheet tends to curl at its corners as a result of the temperature cycle to which it is exposed during heat strengthening. Since the leading edge of the glass sheet has a different thermal history than the trailing edge thereof, the curl at the leading edge corners develops a different configuration from that of the trailing edge corners.
It is more difficult to laminate sheets that have been warped as a consequence of being heat strengthened than to laminate flatter glass sheets having a lower stress pattern that results from annealing. Furthermore, these laminated products may require glass sheets, one or both of which are provided with a coating that reduces the transmission of high energy radiation through the ultimate laminate. The application of heat screening coatings to glass surfaces involves heating the glass to a temperature at which its major surfaces are likely to soften. The heat strengthened glass, regardless of whether it is merely strengthened to improve the resistance of the ultimate product to penetration by foreign objects or whether the sheets are heated in the process of applying a coating to their major surface, develops a warped surface that makes it more difficult to laminate the heat strengthened glass sheets to opposite sides of an interlayer. Glass sheets are easier to laminate when they are oriented to one another so that their top and bottom major surfaces face in the same directions and their leading and trailing edges are aligned and registered.
An interlayer suitable for use in the fabrication of laminated safety glass is preferably a plasticized polyvinyl acetal. Polyvinyl butyral plasticized with triethylene glycol di-2-ethyl butyrate (popularly called "3GH") is a common interlayer. Sheets of such interlayer material are usually supplied with surfaces that are patterned. The patterns on the major surfaces of the interlayer are such as to provide passageways for the removal of entrapped vapors such as air or moisture. The grooves in the patterns are removed by the high pressure that takes place in the autoclave.
It has been suggested that glass sheets be laminated to interlayers by immersing an assembly of alternate sheets of glass and interlayer material within a bath containing a plasticizer and allowing the plasticizer to help unite the interfacial surfaces by the application of heat, although if no hurry is indicated, lamination could take place over an extended period of time at temperatures on the order of room temperatures. Such a technique made it necessary for an operator to wash excess plasticizer from the outer surfaces of the resulting laminated unit. In addition, laminated units produced from units assembled within a bath often had bubbles, which detracted from the commerical value of the units so produced.
Nevertheless, the need exists for a relatively inexpensive process of fabricating transparent, laminated units for relatively small orders of custom sizes that make it impractical to use expensive autoclave equipment.
2. Description of Patents of Interest
British Pat. No. 355,604 to Newtex Safety Glass Co. discloses a method of making a laminate containing a sheet of cellulose acetate between two sheets of glass. A first sheet of glass is laid into a suitable bath of a resin in the A stage of its manufacture. A cellulose acetate sheet is laid on the glass sheet within the bath, and the second glass sheet is laid over the cellulose acetate in the bath. The resultant assembly is removed from the bath, excess bath is wiped off from the outer surfaces of the assembly and the surplus bath composition is removed from between the layers by gentle hand pressure. The resultant assembly is placed in a suitable press and pressed at a pressure of 200 pounds per square inch and at a temperature of 90.degree. to 130.degree. centigrade for forty minutes. This immersion is alleged to avoid areas of non-adhesion between adjacent layers of the assembly. This patent requires waiting for all the layers to be assembled before hand pressure is applied. Therefore, some bubbles that form during assembly cannot be removed. In addition, this patent requires autoclave pressures to accomplish acceptable results.
U.S. Pat. No. 3,449,184 to Balk eliminates the need for an expensive autoclave to complete the lamination of laminated glass products by immersing alternate sheets of glass and interlayer material in a bath of liquid plasticizer material to inundate the sheets in the bath and arranges the sheets in laminate relation to one another while immersed within the bath. Allegedly this process may be accomplished in anywhere from one half hour to several hours at room temperature but the process may be accelerated by subjecting products being cured to heat. Unfortunately, the quality of articles so produced does not always meet the commercial standards required for present day products because of the presence of bubbles. It is impossible to remove such bubbles while the assembly remains immersed within the bath, and, like the Newtex patent, has great difficulty in removing certain bubbles when bubble removal attempts are started only after the assembly is removed from the bath. In addition, working in a bath of plasticizer is very messy and requires the removal of plasticizer from the outer surfaces of the unit when the latter is removed from the bath.
It is believed that the bubbles that form in the laminated products produced by the aforesaid reference patents result from eddy currents that take place during the assembly of sheets within a bath. These eddy currents, which cannot be controlled, sometimes cause bubbles in the film between the interfacial surfaces of the adjacent sheets of the assembly in the laminate. When the individual layers are assembled within a bath of plasticizer material, it is difficult to observe whether such bubbles are present. Hence, it is difficult to determine whether such bubbles require removal from the interfacial surfaces of the assembly in sufficient time to allow lamination to take place with the complete avoidance of bubbles, a necessity when complying with the requirements for present day commerical products. Furthermore, there is no known way of removing an interfacial bubble from an assembly while the assembly is immersed within a bath of plasticizer material during the layup of its components.